Sound transmitting apparatus



June 28, 1938. 5. WURDEL 2,121,941

sounn 'rmmsmmme APPARATUS Filed June 12, 1935 Patented June 28, 1938 UNITED STATES 2,121,941 PATENT OFFICE SOUND TRANSMITTING APPARATUS Giinther Wurdel, Berlin, Germany, assignor to Apparatebau Gesellschaft Neumann & Borm Kommanditgesellschaft, Berlin, Germany imman- June 12, 1935, Serial No. 26,271

In Germany August 25, 1934 I 10 Claims. (01. ria-ms) 10 of the person speaking.

A further object of the invention is to provide a microphone free from chattering effect and having a practically straight characteristic which is devoid of resonance in the audible range.

15 By chattering effect is meant the reverberatory effect or non-linear distortion which arises in the case of ordinary telephone microphones due to the fact that, owing to the mechanical natureandv disposal of the diaphragm, the movement of the.

i diaphragm is not equal on both sides.

A still further object is to provide a telephone in which the effective surface of the microphone, when the telephone is in use, is situated in a plane passing through the corner of the mouth of the speaker.-

A further object is to make provision for excitation of the effective surface of the microphone in immediate fashion by the spoken words.

These and other objects and advantages of the invention will become apparent as the description proceeds.

. It has already been proposed to employ in rooms in which there is a considerable volume of noise laryngeal microphones, which receive the speech vibrations by'being placed against the larynx or the lower jaw of the speaker. Even with microphones of this kind, however, it is mereyl possible to a limited extent to preclude the effect of interfering noises. The articulation is also unsatisfactory, because the higher speech oscillations, which are particularly important for proper intelligibility, are, despite the location of the microphone against the larynx or the jaw,

absorbed by the intermediately disposed skin,

which acts as a cushion, and in consequence do not wholly reach the microphone. Moreover in numerous spokensounds there is no appreciable vibration on the part of the jaw or the larynx,

Any attempt to overcome the strong drop in the alternating potentials delivered by the laryngeal microphone in the range of the higher speech oscillations by the inclusion of rectifying elements between the microphone and the sound reproducing means does also not lead to the de- 55 sired'res ult, as the drop in the directiontowards the higher speechoscillations'does not take place evenly, i. e., in a straight line, but is disturbed by the fact that in'the case of numerous spoken sounds, particularly sibilant sounds, there is no conduction of the sound on the part of the body,

whilst in other sounds an exaggerated emphasis on the initial part of the sound causesa complete distortion of the tone. To avoid the disadvantages associated with laryngeal microphones attempts have been made 5 to employ microphones which are excited by sound in the air, and in which there are provided special auxiliary means, for example screening or damping means, which have the object of withholding interfering noises from the micro-.

phone. Screening and damping means of this kind certainly cause a decrease of the interfering noises at the effective microphone surface, but at the same time they also nullify a part of the voice energy to a similar extent. Moreover when using the microphones hitherto .usual for telephonic purposes it is impossible to hold the same as close to the mouth as would be desirable, as owing to the greatly increased voice energy impinging on the surface of the microphone the distortion of the speech would in this case be so considerable as to render intelligibility impossible. These distortions are caused by the fact that the microphones, when a certain volume of sound is exceeded, are overmodulated. A distortion of the speech also occurs when the microphone is approached too closely towards the mouth because in the case of the carbon microphones hitherto used for telephonic purposes the particles of carbon do not, as 'a result of the very high exciting intensity, oscillate properly about a fixed position ofequilibrium but are thrown out of their original position and do not again return to the same. There then occurs .what I have termed a chattering effect.

Assuming an attempt is made so to construct the damping means above referred to that the overmodulation or the chattering effect of the microphone is eliminated, this will also not lead a to the desired result, as the frequency character- 40 istic of the microphone is not disposed in a straight line but possesses powerful resonance points at numerous points-of the audible range. Generally speaking, interfering noises are not confined to a narrow section of the entire sound range. Accordingly, at resonance points of the microphone affected by corresponding interfering frequencies, the alternating potentials delivered points within the entire audible range. These microphones accordingly reproduce the voice energy passed to the same without any appreciable distortion. Their frequency characteristic, therefore, is disposed practically in a straight line, i. e., is devoid of resonance points. These microphones, which may be constructed as contact I or also condenser or electro-dynamic sound transmitters have heretofore been employed for radio transmitters, the making of talking machine records and for sound-film purposes, as in these cases it is necessary to attain a very natural reproduction of speech and music, which would be impossible with the carbon microphones employed in the telephonic. art. Microphones having a good frequency characteristic have heretofore not been employed at all for telephonic purposes, primarily on account of their small sensitivity, which renders necessary an amplification of the alternating potentials supplied by them. It is quite readily possible, without detriment to the good properties of the same, to construct these microphones in a small size suitable for the purposes of the present invention. 0f the microphones of good frequency characteristic above referred to the contact transmitter possesses a comparatively good sensitivity.

Now it has been ascertained that microphones of good frequency characteristic also have no chattering effect even if-in departure from the mode of use hitherto usual-the effective surface of the microphone is approached as close as possible towards the mouth, where the sound output of the speech possesses its maximum value. This necessitates that the microphone, also in contradistinction to the sound transmitters previously employed for telephonic pu poses, does not have a mouthpiece or the like. Heretofore it has been the general impression that a mouthpiece of the elongated or trumpet variety could not be dispensed with as this served to withhold at least in part the interfering noises and to exert a favorable effect on the frequency characteristic of the microphone. If, however, in the case of the microphone of straight frequency characteristic employed according to the invention a trumpet or similar mouthpiece were employed, the column of air enclosed within the mouthpiece and acting as resonator would also be caused to oscillate by some frequency of the interfering noise. The interfering noise audible in the receiver would accordingly assume a different character in tone, and would be much stronger than would correspond with the intensity of the sound at the sound inlet aperture.

It is known in itself in the acoustical art thatthe sound output increases approximately according to a quadratic function with approach towards the sound source. It has, however, not

been appreciated heretofore that the'sound out-' phone according to the invention in actual use.

Figs. 3 and 4 are front and side views of the,

form of embodiment of the telephone according to Fig. 2, Fig. 4 being partly in section.

Figs. 5 and 6 are front and side views of a modified form of embodiment of the telephone.

In the diagram according to Fig. 1 the curve a-b shows in millivolts the alternating potential delivered by a. microphone approached more and more towards the source of sound, 1. e., dependent on the distance, measured in millimetres, of a source of sound with constant sound energy from the microphone. The broken line 0-0. indicates the intensity of an interfering noise in the same room, this intensity remaining the same at the point of the effective surface of the microphone. a

It is shown by this diagram that the sound energy in the range of the usual spacing of the effective surface of the microphone from the source of sound, 1. e., in the range of approximately 40 to '70 mm., is completely covered by the interfering energyie-d). If the source of sound is greatly approached towards the microphone, i. e., to within approximately 6 mm., the alternating potential delivered by the microphone increases in the case of the example shown in the diagram to more than ten times the value of the interfering potential. The steep ascent of the curve a-b in the case of small spacial disposal provides a certain indication of the unusual extent to which the potential delivered by the microphone will rise when the microphone isapproached even more towards the sound source, 1. e., is moved immediately up to the latter.

It will accordingly be obvious from the above that the approach of the effective surface of the microphone of good, i. e., straightfrequency characteristic employed according to the invention immediately up to the mouth of the person speaking is of decisive importance. Since the ratio between voice energy and interfering energy at the effective surface of a microphone situated immediately against the source of sound is exceptionally favorable. particular precautions do not require to be taken to withhold the interfering noise from the microphone. Practical experiments have shown that with the apparatus according to the invention proper and faultless intelligibility is capable of being obtained even if the unprotected ear is acted upon even to the extent of a sensation of pain.

It has also been found that particular advantage is obtained if the current of air produced upon speaking is not allowed to impinge vertically on the surface of the microphone situated immediately in front of the mouth and thus exert a detrimental effect on the vibrations. According to the invention, therefore, the effective surface of the microphone arranged in the immediate vicinity of the source of sound is disposed approximately parallel to the direction of speech, 1. e., to the direction of movement of the current of air formed upon speaking. With this arrangement the current of air is more or less able to sweep past the surface of the microphone, so that it is then unable appreciably to distort the alternating potentials delivered by the microphone. Preferably the holding member for the microphone is so constructed that the effective surface of the microphone when the receiver is placed against the ear is situated at the corner of the mouth.

As shown, for example, by Fig. 2, the effective microphone surface 2, which is located in a protective casing I,is situated, when the holding member 3 with the receiver 4 is lifted to the ear,- ,very' close to the comer of the mouth of the torboth the transmitter l, 2 as well as the receiver 4. Naturally, as regards the invention,

' it'is quite immaterial whether the microphone and the receiver are united in one holder, or whether the holder,- which may be placed for example about the head,.carries merely. the transmitter I, 2. The essential feature is that the surface of the microphone 2 is situated in the immediate vicinity of the source of sound.

To be able to adapt the requisite small spacing of the effective microphone surface according to the invention to diiferentsizes of head the transmitter is secured to the holding member 3 so as to be adiustable in the plane of the microphone surface 2. I

In the form of embodiment according to Figs. 2 to 4 this adjustability is obtained by an arm 5, which is eccentric and mounted to be rotatable in a corresponding abutment 8 on the holding member. In this way it is a simple matter to adjust the transmitter in such fashion in relation to the holding member in the plane of the microphone surface as to ensure the necessary minimum spacing of the microphone surface from the source of sound.

In the form of embodiment according to Figs.

annexed claims without departing from the spirit of the invention.

What I claim as new and desire to secure by Letters Patent is:

1. A telephone transmitting and receiving set comprising a rigid holder, a receiver mounted at one end thereof and an abutment carried by the other end of said holder, a microphone casing having an arm eccentrically flxed 'thereto,

said arm being rotatable on said abutment, and a microphone of relatively low sensitivity mounted in said casing. f

2. The device as claimed in claim 1, in which the plane of the effective surface of the microphone is always parallel to the effective surface of the receiver.

3. The device as claimed in claim 1, wherein the microphone casing arm extends angularly from the casing.

' straight frequency characteristic which is devoid -of resonance in the audible range, adjustably mounted at the other end of the holder, the construction and arrangement being such that when the sound. receiving apparatus is held to the ear.

the efiective surface of the microphone may lie The microphone, therefore, as shown at the corner of the mouth of the user and in a plane parallel to the breath currents of the user.

5. In a telephone transmitting and receiving set, a non-mouthpiece microphone casing and a microphone free of chattering eflzect and having a practically straight-line frequency characteristic in the audible range, the said microphone being mounted forwardly in the said casing, and the construction and arrangement being such that the. effective surface of the microphone is movably disposed in the immediate vicinity of the speaker's mouth to positions substantially parallel to the direction of speechwhen the telephone-is in use. Y

6. In a telephone transmitting and receiving set, a non-mouthpiece. microphone casing, a microphone free of chattering eifectand having a practically straight-line frequency characteristic in the audible range, the said microphone being mounted with its effective surface forwardly in the said casing substantially parallel to the direction of speech when the telephone is in use, and means for moving the effective surface of the said microphone while maintaining the eifective sur-,

face thereof in the same plane to a position in the immediate vicinity of the speaker's mouth irrespective of the size of the head of the speaker.

L'A sound transmitting device for telephone systems intended for use more particularly under noisy conditions, comprising a microphone, free from chattering effect and having a practically straight frequency-characteristic, devoid of resonance in the audible range, said microphone being adjustably mounted in a telephone set, so that when the receiver is applied to the ear, the effective surface of the said microphone may be moved in a plane of the surface thereof so as to be situated in the immediate vicinity of the speakers mouth at one of the'corners thereof.

8. A sound-transmitting device according to claim 7, in which the effective microphone surface arranged in the immediate vicinity of the source of speech is located in a plane, which is directed parallel 'or approximately parallel to the direction of movement of the current of air resulting upon the formation of the spoken sounds.

9. A receiving and transmitting set for telephones, intended for use more particularly under noisy conditions, comprising a stock, a receiver mounted therein, an arm mounted in said stock,

a microphone free from chattering effect and having a practically straight frequency characteristic, devoid of resonance in the audible range, mounted in said arm the axis of said microphone being eccentric to the arm's axis of rotation and V the plane of the'surface of said microphone parallel to the face of the said receiver.

10. A telephone transmitting and receiving set,

comprising a holder,.a sound receiving apparatus plane parallel to the breath currents of the user.

GiiN'rrmR. 'WURDEL. 

